Pub Version async_phase_notifier CI codecov

A variant of ValueNotifier that has AsyncPhase representing the initial / waiting / complete / error phases of an asynchronous operation.

AsyncPhaseNotifier + AsyncPhase is similar to AsyncNotifier + AsyncValue of Riverpod.

Unlike AsyncNotifier and AsyncValue, which are tied to package:riverpod, AsyncPhaseNotifier and AsyncPhase have no such binding. The notifier can be used as just a handy variant of ValueNotifier with AsyncPhase as its value and convenient methods for manipulating the phases.

Sample apps

Usage

update()

The update() method of AsyncPhaseNotifier executes an asynchronous function, updates the value of AsyncPhaseNotifier automatically according to the phase of the asynchronous operation, and notifies the listeners of those changes.

  1. The value of the notifier is switched to AsyncWaiting when the operation starts.
  2. The change is notified to listeners.
  3. The value is switched to either AsyncComplete or AsyncError depending on the result.
  4. The change is notified to listeners.
final notifier = AsyncPhaseNotifier(0);
notifier.update(() => someAsyncOperation());

updateOnlyPhase()

This is the same as update() except that updateOnlyPhase() only update the phase itself without updating value.data, whereas update() updates both the phase and value.data.

This method is useful when it is necessary to update the phase during execution but the callback result should not affect the data.

e.g. Indicating the waiting status on the UI or notifying the phase change to other parts of the code, with the existing data being kept unchanged.

AsyncPhase

The value of AsyncPhaseNotifier is either AsyncInitial, AsyncWaiting, AsyncComplete or AsyncError. They are subtypes of AsyncPhase.

AsyncPhase provides the when() and whenOrNull() methods, which are useful for choosing an action based on the current phase, like returning an appropriate widget.

child: phase.when(
  initial: (data) => Text('phase: AsyncInitial($data)'), // Optional
  waiting: (data) => Text('phase: AsyncWaiting($data)'),
  complete: (data) => Text('phase: AsyncComplete($data)'),
  error: (data, error, stackTrace) => Text('phase: AsyncError($data, $error)'),
)

async_phase is a separate package, included in this package. See its document for details not covered here.

value.data vs data

data is a getter for value.data. The former is handy and more type-safe since the return type is non-nullable if the generic type T of AsyncPhaseNotifier<T> is non-nullable, whereas tha latter is always nullable, often requiring a null check or a non-null assertion.

Listening for phase changes

listen()

With listen(), you can trigger some action when the phase or its data changes.

This is not much different from addListener(), except for the following points:

  • Returns a function to easily stop listening.
  • Takes a listener function that receives the phase at the time of the call.
  • The listener is called asynchronously because this method uses Stream internally.
final notifier = AsyncPhaseNotifier(Auth());
final cancel = notifier.listen((phase) { /* Some action */ });

...

// Remove the listener if it is no longer necessary.
cancel();

listenFor()

With listenFor(), you can trigger some action in one of the callbacks relevant to the latest phase when the phase or its data changes.

Note:

  • All callbacks are optional.
    • Listener is not added if no callback function is passed.
  • The onWaiting callback is called when the phase has changed to AsyncWaiting and also from AsyncWaiting. A boolean value is passed to the callback to indicate the start or end of an asynchronous operation.
final notifier = AsyncPhaseNotifier(Auth());
final cancel = notifier.listenFor(
  onWaiting: (isWaiting) { /* e.g. Toggling an indicator */ },
  onComplete: (data) { /* e.g. Logging the result of an operation */ }, 
  onError: (e, s) { /* e.g. Showing an error dialog */ },
);

...

// Remove the listener if it is no longer necessary.
cancel();

AsyncPhaseListener

It is also possible to use the AsyncPhaseListener widget to listen for phase changes.

child: AsyncPhaseListener(
  notifier: notifier,
  onWaiting: (isWaiting) { /* e.g. Toggling an indicator */ },
  onComplete: (data) { /* e.g. Logging the result of an operation */ },
  onError: (e, s) { /* e.g. Showing an error dialog */ },
  child: ...,
)

Please note that a listener is added per each AsyncPhaseListener, not per notifier. If this widget is used at various places for one certain notifier, a single notification causes each of them to run its callback function.

Examples

Here is WeatherNotifier extending AsyncPhaseNotifier. It fetches the weather info of a city and notifies its listeners.

class WeatherNotifier extends AsyncPhaseNotifier<Weather> {
  WeatherNotifier() : super(const Weather());

  final repository = WeatherRepository();

  void fetch() {
    update(() => repository.fetchWeather(Cities.tokyo));
  }
}

final notifier = WeatherNotifier();
notifier.fetch();

The examples below use this notifier and show a particular UI component corresponding to each phase of the fetch.

With ValueListenableBuilder

@override
Widget build(BuildContext context) {
  return ValueListenableBuilder<AsyncPhase<Weather>>(
    valueListenable: notifier,
    builder: (context, phase, _) {
      // Shows a progress indicator while fetching and
      // either the result or an error when finished.
      return phase.when(
        waiting: (weather) => const CircularProgressIndicator(),
        complete: (weather) => Text('$weather'),
        error: (weather, e, s) => Text('$e'),
      );
    },
  ); 
}

Or you can use AnimatedBuilder / ListenableBuilder in a similar way.

With Provider

ValueListenableProvider<AsyncPhase<Weather>>.value(
  value: notifier,
  child: MaterialApp(home: ...),
)

@override
Widget build(BuildContext context) {
  final phase = context.watch<AsyncPhase<Weather>>();

  return phase.when(
    waiting: (weather) => const CircularProgressIndicator(),
    complete: (weather) => Text('$weather'),
    error: (weather, e, s) => Text('$e'),
  );
}

With Grab

void main() {
  runApp(
    const Grab(child: App()),
  ); 
}

@override
Widget build(BuildContext context) {
  final phase = notifier.grab(context);

  return phase.when(
    waiting: (weather) => const CircularProgressIndicator(),
    complete: (weather) => Text('$weather'),
    error: (weather, e, s) => Text('$e'),
  );
}

TODO

  • Add API documents
  • x Write tests

Libraries

async_phase
async_phase_notifier